Physics · 5.2.3 Radioactive decay · Half-life
Half-life. Decay it.
Watch a sample of unstable nuclei decay at random. Although individual decays are unpredictable, the count rate halves every half-life. Measure the half-life from the decay curve, just like the dice/cube analogy experiment.
0625 Topic 5.2.3 — Half-life
Random decay · count rate
Background radiation
0.0 s
Undecayed 1000 · Activity 0 /s
Variables
6
1000
5
Live readouts
Undecayed nuclei N
1000
Fraction remaining
100%
Activity (corrected)
— /s
Half-life from graph
— s
Decay is random: you cannot predict which nucleus decays, but N halves every half-life. Subtract the background count first.
Decay curve — count rate vs time
📋 Method (Cambridge / dice analogy)
- Measure the background count rate first (source removed) and subtract it from every reading.
- Record the corrected count rate from a Geiger–Müller tube at regular time intervals.
- Plot corrected count rate against time — a smooth decay curve.
- Read off the time for the count rate to halve (e.g. from 800 → 400). Check it halves again in the same time. This is the half-life.
Dice analogy: roll many dice, remove every "six" each throw — the number left falls by a fixed fraction each round, modelling random decay.
⚠ Precautions
- Always correct for background radiation.
- Take several half-life intervals and average for reliability.
- Handle sources with tongs, keep at arm's length, minimise exposure time.
🎯 Syllabus reference (0625)
- 5.2.3 Half-life — define half-life; explain that decay is random and spontaneous; determine half-life from data or a decay curve; correct measured count rate for background radiation.